Many installations in the UK are connected to the public highway by estate roads, which are in fact private property, but look to all intents and purposes indistinguishable from any other road.
The difference is that there will be, somewhere along their length, a 1959 Highways Act non-dedication notice which prevents the acquisition of public rights, and retains their ability to manage, patrol and close these routes if they so wish - or just to be officious which seems to be what you have encountered